The subject matter of the present invention is a method for the non-destructive inspection of a test object of great material thickness by means of ultrasound, the use of a test probe known per se for carrying out the method, an ultrasonic test probe particularly suitable for carrying out the method, a control unit for an ultrasonic test probe for the non-destructive inspection of a test object of great material thickness by means of ultrasound, which can, in particular, be configured for carrying out the method, and a device for the non-destructive inspection of a test object of great material thickness by means of ultrasound.
A variety of methods for the non-destructive inspection of a test object by means of ultrasound are known from the field of material testing. In the pulse echo methods, a short ultrasonic pulse generated by an ultrasonic transducer acting as a transmitter is suitably insonified into a test object so that it propagates in the test object. If the pulse hits a flaw in the test object, for example a discontinuity operated as a receiver, or a geometric structure, the pulse is reflected at least partially. The reflected pulse is detected by means of an ultrasonic transducer. An ultrasonic transducer is frequently used both as a transmitter as well as a receiver. The position of the discontinuity in the test object can be deduced from the travel time between the insonification of the pulse into the test object and the arrival of the reflected pulse at the receiver. The amplitude of the reflected pulse can be used to obtain information on the size of the discontinuity.
In standardized manual ultrasonic testing, two methods for assessing the size of a discontinuity have become established globally, i.e. the reference body method (also referred to in short as DAC method, from “distance-amplitude correction”) and the DGS method (from “distance-gain-(flaw)size”). Both methods are different as regards application, but not with respect to the fundamental physics of sound propagation and sound reflection they are based on. In both methods, the examiner determines the size (diameter) of a model reflector (cylindrical reflector in the DAC method, circular disk in the DGS method). The size thus determined is, in principle, not identical to the actual flaw size; it is therefore referred to as equivalent circular disk or cross hole diameter. In case of the usage of circular disk reflectors the shorter term “equivalent reflector size” (ERS) has become established. That the actual flaw size does not correspond to the equivalent reflector size is due to the fact that the portions of the sound reflected by a natural flaw are additionally affected by the shape, orientation and surface properties of the flaw. Because further examinations in this respect are difficult and not very practicable in manual ultrasonic testing, the criteria for recording faults are tied to a certain equivalent reflector size in most specifications and guidelines for ultrasonic testing. This means that the examiner determines in practice whether a detected fault reaches or exceeds the equivalent reflector size specified as a threshold value (recording threshold) in the documentation. Beyond this, he generally will have to carry out further inspections, for example with regard to the recording length, echo dynamics etc., which, however, shall not be discussed here.
The laws of sound propagation in matter have long been theoretically known and were verified in practice by many experiments. The development of the modern assessment methods presents to ways: In the DAC method, the characteristics of the sound field are determined in each case prior to ultrasonic testing by means of a calibration measurement on a reference body; in contrast, the DGS method uses the theory in the form of the so-called DGS diagrams provided for the test probes. A DGS diagram shows the echo amplitudes of circular disk reflectors of different diameters and of a large flat reflector (back face) as a function of the distance.
For carrying out the DAC method, a reference body with one or more reference reflectors that corresponds to the test object must be provided for inspection. The dependency on distance of the echo amplitudes is experimentally determined by means of the bore holes present in the reference body, and transferred onto the display of the testing device as a curve. This curve automatically contains all influences of the test probe (sound field) and of the material. The test object can now be scanned with the ultrasonic test probe. A recordable indication is given if an echo reaches or exceeds the DAC curve.
The requirement for carrying out the DGS method is that the corresponding ultrasonic test probe-specific DGS diagram has to be provided for the ultrasonic test probe which is to be used for the inspection task. The equivalent reflector size is determined from the DGS diagram for the maximum echo of a detected flaw indication. It is thus possible to assess whether the indication is recordable or not.
The use of microprocessor-controlled ultrasonic devices results in considerable simplifications in both assessment methods, which lead to time being saved and a higher testing reliability. In particular, DGS assessment is simplified in a modern ultrasonic device by storing the DGS diagrams of standard test probes in the device. Preferably, a flat bottom hole (circular disk), a cross hole or a back face can be selected as a reference reflector. An evaluation program can provide for an immediate assessment of a detected flaw indication. In the process, the recording threshold exceedance, i.e. the dB value by which the flaw indication exceeds the predetermined recording curve, can be directly displayed on a display device, e.g. an integrated screen. This form of evaluation corresponds to the practice of most testing standards. These include, for example the well-known DIN EN 583-2, DIN 54 125 and SEL 072, but also all other specifications in which flat bottom holes are prescribed as reference reflectors.
Even though the above-mentioned pulse echo methods have been well-established methods in the field of material testing for years, their application to the inspection of test objects with a great material thickness, e.g. in the case of thick-walled pressure or safety tanks with a great wall thickness, today still requires a lot of effort. In the case of the DGS method, the reason for this is that an ultrasonic transducer generates a sound field which generally has several local sound pressure maxima located on the acoustic axis. The position of the last sound pressure maximum on the acoustic axis is in this case referred to as “near-field length N”. For an ultrasonic transducer having a circular active surface with a diameter D (hereinafter referred to as “circular ultrasonic transducer”), the near-field length N is given by:
  N  =            D      2              4      ⁢      λ      
In this case, λ is the wavelength of the sound field in the test object material. In practice, it was found that a reliable echo evaluation is possible only if, viewed along the sound path, the distance of the defect from the insonification location is at least 70% of the near-field length N of the test probe used. If, for example, an ultrasonic test probe with a circular transducer is used, the latter must therefore have a small diameter D in order to assess defects close to the surface.
On the other hand, the sensitivity of a test probe decreases rapidly when the distance between the coupling location and the position of the flaw is doubled, owing to the divergence of the sound field propagating in the test object material. In the case of a ultrasonic test probe with a circular transducer, the divergence observed is substantially determined by the diameter D of the transducer, the rule being that large transducers have a smaller divergence than small ones. Because most testing standards for a size assessment of a flaw prescribe a minimum indication level, this results in the necessity, when inspecting test objects with a great material thickness, of using test probes with a large transducer diameter D in the inspection of sectors having a large distance from the coupling location, i.e. for the detection of deep flaws. However, they are not suitable for detecting defects close to the surface. In practice, a plurality of different test probes is therefore always used in the inspection of test objects with a great material thickness, such as, for example thick-walled cast containers or of long shafts. If a flaw is identified, an ultrasonic test probe is specifically selected for a quantitative flaw determination whose near-field length N is approximately in the range of the distance between the coupling location and the flaw position, with the above-mentioned criterion for the near-field length N having to be observed. A consequence of this is that, on the one hand, a plurality of different test probes has to be kept in storage, which increases the technical expenditure, on the other hand, a change of the test probe increases the inspection expenditure, which leads to cost disadvantages.